Plagioclase is a series of tectosilicate minerals within the feldspar group. Rather than referring to a mineral with a specific chemical composition, plagioclase is a continuous solid solution series. This was first shown by the German mineralogist Johann Friedrich Christian Hessel in 1826, the series ranges from albite to anorthite endmembers, where sodium and calcium atoms can substitute for each other in the minerals crystal lattice structure. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or record-groove effect, plagioclase is a major constituent mineral in the Earths crust, and is consequently an important diagnostic tool in petrology for identifying the composition and evolution of igneous rocks. Plagioclase is a constituent of rock in the highlands of the Earths moon. Analysis of thermal emission spectra from the surface of Mars suggests that plagioclase is the most abundant mineral in the crust of Mars, the extinction angle is an optical characteristic and varies with the albite fraction.
There are several named plagioclase feldspars that fall between albite and anorthite in the series, the following table shows their compositions in terms of constituent anorthite and albite percentages. Anorthite was named by Gustav Rose in 1823 from the Ancient Greek meaning oblique, anorthite is a comparatively rare mineral but occurs in the basic plutonic rocks of some orogenic calc-alkaline suites. Albite is named from the Latin albus, in reference to its pure white color. It is a common and important rock-making mineral associated with the more acid rock types and in pegmatite dikes, often with rarer minerals like tourmaline. The intermediate members of the group are very similar to each other. Bytownite, named after the name for Ottawa, Canada, is a rare mineral occasionally found in more basic rocks. Labradorite is the characteristic feldspar of the basic rock types such as diorite, andesite. Labradorite frequently shows an iridescent display of colors due to light refracting within the lamellae of the crystal and it is named after Labrador, where it is a constituent of the intrusive igneous rock anorthosite which is composed almost entirely of plagioclase.
A variety of known as spectrolite is found in Finland. Andesine is a mineral of rocks such as diorite which contain a moderate amount of silica. Oligoclase is common in granite, syenite and gneiss and it is a frequent associate of orthoclase. The name oligoclase is derived from the Greek for little and fracture, sunstone is mainly oligoclase with flakes of hematite
Gneiss is a common distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. The foliations are characterized by alternating darker and lighter colored bands, the word gneiss comes from the Middle High German verb gneist. It has occurred in English since at least 1757, gneissic rocks are usually medium- to coarse-foliated, they are largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, gneiss rocks may be named after a characteristic component such as garnet gneiss, biotite gneiss, albite gneiss, etc. Orthogneiss designates a gneiss derived from a rock, and paragneiss is one from a sedimentary rock. Gneissose rocks have properties similar to gneiss, gneiss appears to be striped in bands, called gneissic banding. The banding is developed under high temperature and pressure conditions, the minerals are arranged into layers that appear as bands in cross section.
The appearance of layers, called compositional banding, occurs because the layers, the darker bands have relatively more mafic minerals. The lighter bands contain relatively more felsic and these forces stretch out the rock like a plastic, and the original material is spread out into sheets. Another cause of banding is metamorphic differentiation, which separates different materials into different layers through chemical reactions, not all gneiss rocks have detectable banding. In kyanite gneiss, crystals of kyanite appear as random clumps in what is mainly a plagioclase matrix, henderson gneiss is found in North Carolina and South Carolina, US, east of the Brevard Shear Zone. It has deformed into two sequential forms, the second, more warped, form is associated with the Brevard Fault, and the first deformation results from displacement to the southwest. Most of the Outer Hebrides of Scotland have a formed from Lewisian gneiss. In addition to the Outer Hebrides, they form basement deposits on the Scottish mainland west of the Moine Thrust and on the islands of Coll and Tiree.
These rocks are igneous in origin, mixed with metamorphosed marble and mica schist with intrusions of basaltic dikes. Gneisses of Archean and Proterozoic age occur in the Baltic Shield, in antiquity, gneisses were utilized in architectural construction. They were used to erect the Sphinx of Taharqo in the Nile Valley, list of rock types Blatt and Robert J. Tracy. Petrology, Igneous and Metamorphic, 2nd ed. Freeman, mcKirdy, Roger Crofts and John Gordon
Monzonite is an igneous intrusive rock. It is composed of equal amounts of plagioclase and alkali feldspar. It may contain minor amounts of hornblende and other minerals, if quartz constitutes greater than 5%, the rock is termed a quartz monzonite. If the rock has a percentage of alkali feldspar, it grades into a syenite. With an increase in calcic plagioclase and mafic minerals the rock becomes a diorite. The volcanic equivalent is the latite, monzonite was originally named after the Monzoni range in Val di Fassa where it is abundant. As rock definitions have been systematized and codified, this association has lost any relevance to the rocks definition
Quartz monzonite or adamellite is an intrusive, igneous rock that has an approximately equal proportion of orthoclase and plagioclase feldspars. It is typically a light colored phaneritic to porphyritic granitic rock, the plagioclase is typically intermediate to sodic in composition, andesine to oligoclase. Quartz is present in significant amounts, biotite and/or hornblende constitute the dark minerals. Because of its coloring, it is confused with granite. Rock with less than five percent quartz is classified as monzonite, a rock with more alkali feldspar is a syenite whereas one with more plagioclase is a quartz diorite. The fine grained volcanic rock equivalent of quartz monzonite is quartz latite, Quartz monzonite porphyry is often associated with copper mineralization in the porphyry copper ore deposits. A massive outcrop of igneous rock can be seen on the bald summit of Croydon Mountain near Cornish. Stone Mountain in Georgia is a quartz monzonite monadnock. The large boulders of Joshua Tree National Park in southern California are quartz monzonite, a large pluton in the Atlanta lobe of the Idaho Batholith, near McCall, Idaho, is made of quartz monzonite.
The Guilford Quartz Monzonite and Woodstock Quartz Monzonite, probably comagmatic, are located in central Maryland, media related to Quartz monzonite at Wikimedia Commons
The continental crust is the layer of igneous and metamorphic rocks that forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial because its composition is more felsic compared to the oceanic crust. The continental crust consists of layers, with a bulk composition that is intermediate to felsic. The average density of continental crust is about 2.7 g/cm3, less dense than the material that makes up the mantle. Continental crust is less dense than oceanic crust, whose density is about 2.9 g/cm3. At 25 to 70 km, continental crust is thicker than oceanic crust. About 40% of Earths surface is occupied by continental crust. It makes up about 70% of the volume of Earths crust, because the surface of continental crust mainly lies above sea level, its existence allowed land life to evolve from marine life. There is little evidence of continental crust prior to 3.5 Ga, all continental crust ultimately derives from the fractional differentiation of oceanic crust over many eons.
This process has been and continues today primarily as a result of the associated with subduction. In contrast to the persistence of continental crust, the size, different tracts rift apart and recoalesce as part of a grand supercontinent cycle. There are currently about 7 billion cubic kilometers of continental crust, the relative permanence of continental crust contrasts with the short life of oceanic crust. Because continental crust is less dense oceanic crust, when active margins of the two meet in subduction zones, the oceanic crust is typically subducted back into the mantle. Continental crust is rarely subducted.01 Ga, whereas the oldest oceanic crust is from the Jurassic, Continental crust and the rock layers that lie on and within it are thus the best archive of Earths history. The height of mountain ranges is usually related to the thickness of crust and this results from the isostasy associated with orogeny. The crust is thickened by the compressive forces related to subduction or continental collision, the buoyancy of the crust forces it upwards, the forces of the collisional stress balanced by gravity and erosion.
This forms a keel or mountain root beneath the mountain range, the thinnest continental crust is found in rift zones, where the crust is thinned by detachment faulting and eventually severed, replaced by oceanic crust. The edges of continental fragments formed this way are termed passive margins, igneous rock may be underplated to the underside of the crust, i. e. adding to the crust by forming a layer immediately beneath it
Eclogite is a mafic metamorphic rock. Eclogite forms at pressures greater than typical of the crust of the Earth. An unusually dense rock, eclogite can play an important role in driving convection within the solid Earth, the fresh rock can be striking in appearance, with red to pink garnet in a green matrix of sodium-rich pyroxene. Accessory minerals include kyanite, quartz, coesite, phengite, zoisite, corundum, plagioclase is not stable in eclogite. Eclogite typically results from metamorphism of mafic igneous rock as it plunges into the mantle in a subduction zone. Such eclogites are generally formed from precursor mineral assemblages typical of blueschist-facies or amphibolite-facies metamorphism, eclogite can form from magmas that crystallize and cool within the mantle or lower crust. Eclogite facies is determined by the temperatures and pressures required to metamorphose basaltic rocks to an eclogite assemblage, the typical eclogite mineral assemblage is garnet plus clinopyroxene. Eclogites record pressures in excess of 1.2 GPa at >400–1000 °C and this is high-pressure, medium- to high-temperature metamorphism.
Diamond and coesite occur as trace constituents in some eclogites and record particularly high pressures, in fact, such ultrahigh-pressure metamorphism has been defined as metamorphism within the eclogite facies but at pressures greater than those of the quartz-coesite transition. Some UHP rocks appear to record burial at depths greater than 150 km, the rarity of lawsonite eclogites therefore does not reflect unusual formation conditions but unusual exhumation processes. Lawsonite eclogite is known from the U. S. Guatemala, Australia, the Dominican Republic, eclogite is the highest pressure metamorphic facies and is usually the result of advancement from blueschist metamorphic conditions. Eclogite is a rare and important rock because it is formed only by conditions typically found in the mantle or the lowermost part of thickened crust, eclogite may completely retrogress to amphibolite or granulite during exhumation. Xenoliths of eclogite occur in kimberlite pipes of Africa, Canada, felsic rocks in these terranes contain sillimanite, coesite and pyroxene, and are rare, peculiar rocks formed by an unusual tectonic event.
Peridotite is the dominant rock type of the mantle, not eclogite. Likewise, peridotite is a more important source rock of common magmas. Melting of eclogite to produce basalt directly is not supported in modern petrology. Unreasonably high degrees of melting are required to attain basaltic compositions. To get a basalt from melting an eclogite it has to undergo 100% partial melting, basalts can be modelled as having been produced by 1 to 25% partial melting of peridotite, such as harzburgite and lherzolite
Bowen's reaction series
Bowens reaction series is able to explain why certain types of minerals tend to be found together while others are almost never associated with one another. Based upon Bowens work, one can infer from the present in a rock the relative conditions under which the material had formed. The series is broken into two branches, the continuous and the discontinuous, the branch on the right is the continuous. This is because minerals are most stable in the conditions closest to those under which they had formed, on the other hand, the low temperature minerals are much more stable because the conditions at the surface are much more similar to the conditions under which they formed
Jakob Johannes Sederholm was a Finnish petrologist most associated with his studies of migmatites. Troubled by illness throughout his life, Sederholm originally chose to study geology to allow him to work outdoors, after studying first in Helsinki, in Stockholm and Heidelberg, Sederholm returned to Finland to work for the Geological Survey of Finland. In 1893 he assumed the role of director of this institution, working on local Precambrian basement rocks, Sederholm instigated a map-making programme that, between 1899 and 1925, published many maps and descriptions of their geological history. Gneisses in the areas he studied were often of mixed composition, Sederholm termed these as migmatites, and viewed them as the product of the intrusion of igneous magma into metamorphic rocks at depth. During his career Sederholm received both the Murchison Medal from the Geological Society of London and the Penrose Medal from the Geological Society of America, the mineral sederholmite is named in his honour.
Aside from his work, Sederholm was a member of the Diet of Finland. He was a member and chairman of the Economic Society of Finland, in the 1974 historical novel Centennial, James Michener listed Sederholm among those scientists who made early estimates of the age of the Earth. Sederholms estimate was 40 million years, hackman V. Jacob Johannes Sederholm, Biographic notes and bibliography // Bulletin de la Commission géologique de Finlande. 34 p. Papunen H. Jakob Johannes Sederholm – geologi, complete Dictionary of Scientific Biography,2008. Jakob Johannes Sederholm - Lithos,2010, Murchison Medal winners Penrose Medal winners Sederholmite mineral data Sederholmite mineral data
The Precambrian is the earliest period of Earths history, set before the current Phanerozoic Eon. The Precambrian is a supereon that is subdivided into three eons of the time scale. It spans from the formation of Earth about 4.6 billion years ago to the beginning of the Cambrian Period, about 541 million years ago, the Precambrian accounts for 89% of geologic time. Relatively little is known about the Precambrian, despite it making up roughly seven-eighths of the Earths history, the Precambrian fossil record is poorer than that of the succeeding Phanerozoic, and fossils from that time are of limited biostratigraphic use. This is because many Precambrian rocks have been metamorphosed, obscuring their origins, while others have been destroyed by erosion. A stable crust was apparently in place by 4,412 Ma, the term Precambrian is recognized by the International Commission on Stratigraphy as a general term including the Archean and Proterozoic eons. It is still used by geologists and paleontologists for general discussions not requiring the more specific eon names and it was briefly called the Cryptozoic eon.
A specific date for the origin of life has not been determined, carbon found in 3.8 billion year old rocks from islands off western Greenland may be of organic origin. Well-preserved microscopic fossils of bacteria older than 3.46 billion years have found in Western Australia. Probable fossils 100 million years older have been found in the same area, there is a fairly solid record of bacterial life throughout the remainder of the Precambrian. The oldest fossil evidence from that era of such complex life comes from the Lantian formation of the Ediacarian period, a very diverse collection of soft-bodied forms is found in a variety of locations worldwide and date to between 635 and 542 Ma. These are referred to as Ediacaran or Vendian biota, hard-shelled creatures appeared toward the end of that time span, marking the beginning of the Phanerozoic era. By the middle of the following Cambrian period, a diverse fauna is recorded in the Burgess Shale. The explosion in diversity of lifeforms during the early Cambrian is called the Cambrian explosion of life, while land seems to have been devoid of plants and animals and other microbes formed prokaryotic mats that covered terrestrial areas.
Evidence of the details of plate motions and other activity in the Precambrian has been poorly preserved. It is generally believed that small proto-continents existed prior to 4280 Ma, the supercontinent, known as Rodinia, broke up around 750 Ma. A number of glacial periods have been identified going as far back as the Huronian epoch, one of the best studied is the Sturtian-Varangian glaciation, around 850–635 Ma, which may have brought glacial conditions all the way to the equator, resulting in a Snowball Earth. The atmosphere of the early Earth is not well understood, most geologists believe it was composed primarily of nitrogen, carbon dioxide, and other relatively inert gases, and was lacking in free oxygen
Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means change in form. The original rock is subjected to heat and pressure, causing profound physical and/or chemical change, the protolith may be a sedimentary, an igneous, or even an existing type of metamorphic rock. Metamorphic rocks make up a part of the Earths crust. They are classified by texture and by chemical and mineral assemblage and they may be formed simply by being deep beneath the Earths surface, subjected to high temperatures and the great pressure of the rock layers above it. They can form from tectonic processes such as continental collisions, which cause horizontal pressure and they are formed when rock is heated up by the intrusion of hot molten rock called magma from the Earths interior. The study of rocks provides information about the temperatures and pressures that occur at great depths within the Earths crust. Some examples of rocks are gneiss, marble, schist.
Metamorphic minerals are those that only at the high temperatures and pressures associated with the process of metamorphism. These minerals, known as index minerals, include sillimanite, staurolite and some garnet. Other minerals, such as olivines, amphiboles, micas and quartz, may be found in metamorphic rocks and these minerals formed during the crystallization of igneous rocks. They are stable at temperatures and pressures and may remain chemically unchanged during the metamorphic process. However, all minerals are only within certain limits. The change in the size of the rock during the process of metamorphism is called recrystallization. Both high temperatures and pressures contribute to recrystallization, high temperatures allow the atoms and ions in solid crystals to migrate, thus reorganizing the crystals, while high pressures cause solution of the crystals within the rock at their point of contact. The layering within metamorphic rocks is called foliation, and it occurs when a rock is being shortened along one axis during recrystallization.
This causes the platy or elongated crystals of minerals, such as mica and chlorite and this results in a banded, or foliated rock, with the bands showing the colors of the minerals that formed them. Textures are separated into foliated and non-foliated categories, foliated rock is a product of differential stress that deforms the rock in one plane, sometimes creating a plane of cleavage. For example, slate is a metamorphic rock, originating from shale
Rhyodacite is an extrusive volcanic rock intermediate in composition between dacite and rhyolite. It is the equivalent of granodiorite. Phenocrysts of sodium-rich plagioclase, sanidine and biotite or hornblende are typically set in an aphanitic to glassy light to intermediate-colored matrix, rhyodacite is a high silica rock containing 20% to 60% quartz with the remaining constituents being mostly feldspar. The feldspar is a mix of alkaline feldspar and plagioclase, with plagioclase forming 35% to 65% of the mix, rhyodacite often exists as explosive pyroclastic volcanic deposits
Goniometric measurements made by Gerhard vom Rath in 1873 led him to refer the crystals to the tetragonal system. This pseudo-cubic character of leucite is very similar to that of the mineral boracite, the crystals are white or ash-grey in colour, hence the name suggested by A. G. Werner in 1701, from λευκος, white. The Mohs hardness is 5.5, and the specific gravity 2.47, inclusions of other minerals, arranged in concentric zones, are frequently present in the crystals. On account of the color and form of the crystals the mineral was known as white garnet. This article incorporates text from a now in the public domain, Hugh